Manchester Memoirs, Vol. Ixi. (191 ?) No. 9 11 



Anodic Metal Overvoltages in Solutions of Metallic 



Chlorides. 



Current Copper. Zinc. Tin. Iron. Nickel. Cohalt. 



density. A. B. A. B. A. B. A. B. A. B. A. B. 



2 0.02 0.0;3 0.00 0.00 0.00 0.00 0.05 0.00 0.07 0.07 0.03 0.06 



4 0.03 0.04 0.00 0.00 0.00 0.00 0.07 0.()i2 0.09 0.08 0.02 0.07 



6 0.03 0.04 0.00 0.00 0.00 0.00 0.07 0.03 0.09 0.08 0.00 0.07 



10 0.03 O.Oi 0.00 0.00 0.00 0.00 0.08 0.04 0.09 0.09 0.00 0.06 



20 0.04 0.04 0.00 0.00 0.00 0.00 0.09 0.06 0.08 0.09—0.02 0.06 



50 0.05 0.03 0.01 0.01 0.01 0.01 0.10 0.08 0.08 0.10—0.03 0.05 



100 0.02 - 0.02 0.02 0.02 0.02 0.10 0.09 0.08 0.10—0.02 0.05 



200 — — 0.03 0.03 0.03 0.03 0.10 0.10 0.09 0.10 + 0.03 0.07 



400 — — 0.04 0.04 0.04 0.05 0.11 0.11 0.10 0.10 0.07 0.09 



Cathodic (hydrogen) overvoltage. 



It was previously suggested that overvoltage is due to four 

 factors : — 



1. Supersaturation of the electrode surface witih non-electri- 

 fied gas under high pressures. 



2. Formation of a series of alloys or solid solutions of the 

 discharged ion, or a piroduct of the discharged ion. with the 

 electrode surface. 



3. Deficiency or excess of non-hydrated ions in the imme- 

 diate neighbourhood of the electrodes. 



4. Inductive action of the escaping ionised gas on the 

 electrode. 



A study of thle tables given will soon show that the above 

 theory is inadequate to explain all the observed phenomena. 



An amalgamated zinc cathode shows the remarkably high 

 and constant value of 0.88 volt. If factor (i) above, were the 

 true cause of overvoltage, we should have to assume pressures 

 greater than vcr'^ atmospheres, in the soft surface of this alloy. 

 Such pressures are incredible, and hence this factor cannot be 

 the main cause of overvoltage. The remarkable constancy Of 

 the values for certain electrodes, notably thallium and rhodium, 

 shows that factor (3) can only exert an inappreciable effect upon 

 the total, since the eff'ect, if any, must be proportional to the 

 current density. Factor (3) may therefore be safely omitted 

 from the general theory. 



The very small fall of overvoltage observe(d in most cases 

 iat the highest current densit,ies also shows that factor (4) cannot 

 in general reduce the values by more than 20 or 30 millivolts, 

 and therefore, although the effect is real, it is of little import- 

 ance, specially as these very high current aensitles are seldom 

 or never used in practical work. 



We are left, therefore, with factor (2), or some modification 

 of it, to account for all of the main facts of overvoltage. 



On examining the tables more closely, it is evident that in 

 spite o!f the variations due to time and current density, certain 



